Note: If your project has 5,000 square feet, or greater, of new plus replaced hard surface area, your application must be prepared by a professional engineer. See the “For Professional Engineers” later on this webpage.

If your project requires On-site Stormwater Management or must meet Flow Control or Water Quality Standards, you must first investigate the feasibility of infiltration facilities (such as a dry well) to meet these standards. The exception to this is if your project is located in area that is mapped as "Infiltration Investigation Not Required." The “Infiltration Investigation Not Required” map is available to you as a Base Map Layer on our GIS website. If you are required to do infiltration testing, you must submit the infiltration checklists with your plans. If infiltration is not required, you must indicate the reason on the Site and Drainage Control Summary.

Nutrient Critical Receiving Waters

At the time this Manual was developed, there were no nutrient-critical receiving water segments determined to be impaired due to phosphorus contributed by stormwater. In the future, the City may designate a waterbody as a nutrient-critical receiving water as defined by the SMC, Section 22.801.150.

Reference materials:

Here's a list of documents to help you understand our stormwater code:

In the “Computational Timestep” pull down bar, select the appropriate time step for the type of facility that is being designed (see Table F.12, Appendix F, Section F-4 of the 2016 Seattle Stormwater Manual)

Note: We do not provide technical support for the approved continuous rainfall hydrologic models or in the application of the model to specific projects. Consultants who would like training in the use of the approved models may contact these companies directly: